Packaging trays formed of expanded organic polymer material, such as polystyrene, are used to package a wide variety of food items, from meat to mushrooms, and are usually over-wrapped with a transparent plastic wrap, such as polyvinyl chloride (PVC) shrink-wrap film material, polyethylene chloride (PEC) stretch film, or other similar wrap made of PVC, PEC, or their combination, as well as other commercial shrink wrap or stretch wrap material. Because of different temperature coefficients of expansion between the tray and the wrap material, the wrap tends to loosen following a substantial temperature change. Not only does this provide a poor appearance for goods that must be attractively displayed, but can also result in loss of wrap from the tray with the loss of the contents.
In order to accommodate loosening of the wrap, food packers try to wrap the tray as tightly as possible to the point where the tray walls are bent inwardly so that when the wrap loosens, the outward spring moment of the walls of the tray will maintain tension on the wrap material. However, with packaging trays of standard construction, if the wrap is too tight, tray failure can result by development of side wall cracks or even breakage of the side walls. Such failure can cause the processing equipment to jam. Those trays that survive the processing line are still subject to side wall cracking during handling with resultant looseness of the wrap, thereby defeating the purpose of tightly applying the wrap. Attempting to cure the problem simply by increasing the total thickness of the tray requires greater belt pressure on the sides of the trays in the processing line, with resultant upgrading of capital equipment, and still is not entirely satisfactory in that the resultant tray is often less pliable with lower spring moment so that is does not satisfactorily compensate for loosening of the wrap.
The present invention provides a unique packaging tray construction that overcomes the foregoing drawbacks and permits very tight wrapping of the packaging tray and contents without any increase in belt pressure while maintaining the integrity of the tray walls, resulting in less tray failures during processing and after processing.
The present invention results from the combination of a number of features that normally would be perceived of as unrelated to each other, but synergistically cooperate in the present invention to produce a packaging tray having the aforesaid advantages. As an initial matter, the tray must be rectangular, which can include square shapes, and must have a peripheral edge, features, of course, that are not particularly unique to a packaging tray. What is unique is the combination to be described, a part of which relates to the differential in thickness between the peripheral edge and the major portion of the associated side wall that rises from the bottom panel of the packaging tray. This relationship is defined not only in relative terms but in absolute terms. In a first feature, the peripheral edge of the packaging tray is at least 10% thicker than the cross-sectional thickness of a major portion of the associated wall that terminates in that edge; in a second feature, the peripheral edge has a thickness in the range of 90/1000 inch to 500/1000 inch, preferably 180/1000 inch to 250/1000 inch. In a third feature of the invention, the peripheral edges of the tray's side walls are relieved so as to be curvilinearly interrupted along their lengths at at least two intervals along the length of each side wall peripheral edge, preferably regularly and preferably along at least three intervals. The foregoing combination of features results in a packaging tray having increased strength to enable very tight wrapping while retaining sufficiently high spring moment to maintain a tight wrap and attractive appearance throughout the course of packaging, shipping, and handling.
In a further, particular embodiment, the corners of the packaging containers where the walls meet are formed so as to rise from the bottom panel at a substantially greater angle than the angle at which the walls themselves rise from the panel. While this is not in and of itself a unique feature, its combination with the three features described above enhances the synergistic combination of the other features. Similarly, in another further embodiment, the peripheral side wall edges are cut normal to the plane defined by the bottom panel of the packaging tray. Again, this is not, of itself, unique but its combination with the aforesaid features aids in the synergistic results that are obtained.